Effect Of Zr Content And Solidification Condition On The Existence Form Of Zr And As-cast Structure Of Aluminium Alloy

Trace Zr element added to aluminium alloys can significantly enhance the comprehensive properties of aluminium alloy.The Zr has several kinds of existence forms(solid solution state,binary Al3Zr phase with two crystal styles of L12 and D023)in the aluminium matrix during solidification and heat treatment process.The existence forms of the Zr formed in melting and casting process affects not only the structure and property of the as-cast ingot,but also the final property of the alloys by structure heredity function.So it is important to control the Zr existence form in aluminium matrix during melting and casting process.In this experiment,high-purity aluminum and industrial pure aluminum samples with different Zr content were obtained by changing Zr content and solidification conditions during melting and castingprocess,and the effects of Zr content and solidification conditions on the existence form of Zr and as-cast microstructure of aluminum alloy(high-purity aluminum and industrial pure aluminum)were analyzed.This experiment includes two main contents:(1)the effect of Zr content and solidification conditions on the as-cast structure of high purity aluminum containing Zr element;(2)the effect of Zr content and solidification conditions on the as-cast structure of industrial pure aluminum containing Zr element.The experimental results show that:(1)When casting at 780? and the cooling rate is higher than 5.10?/s,the Zr content is low than 0.20 wt.%can completely dissolve within high-purity aluminum matrix;certain agglomerates of Al3Zr phases with complex morphology were formed in the aluminium matrix as the Zr increased up to 0.20 wt.%.(2)When the Zr content is 0.25 wt.%and cooling rate is reduced to below 6.99?/s,the Al3Zr phase is formed,when the Zr content is 0.30wt.%and cooling rate is reduced to below 36.00?/s,the Al3Zr phase is formed.As the cooling rate decreases,the fraction and the size of the agglomerates of Al3Zr increased.(3)When the solidification conditions are fixed(cooling rate is 5.10?/s,temperature gradient is 0.01?/mm),with the Zr content changed from 0.05 wt.%to 0.15 wt.%,the average grain size remains unchanged at around 500 ?m.The average grain size linearly decreased from 492?m to 159 ?m,as the Zr content increased from 0.15 to 0.25 wt.%.Further increasing of Zr content up to 0.30 wt.%does not lead to significant difference of the average grain size.(4)When casting at 700? and 780?,the Zr?0.20 wt.%,primary Al3Zr phase is not found in industrial pure aluminum samples within 45 mm from the bottom.When the Zr content is 0.30 wt.%,the stripe-like Al3Zr phase is found at different positions of sample(casting at 700?),and the the cooling rate has no effect on its morphology;but casting at 780? agglomerated Al3Zr phase was found in the area with lower cooling rate.(5)When the Zr content is between 0 and 0.30 wt%,the industrial pure aluminum ingot includes the bottom columnar crystal area,top columnar crystal area and the central equiaxed crystal area.With the increase of Zr content,the size of equiaxed grains in the center of ingot increases first and then decreases.At the same Zr content,the grains of the samples casting at 700? are relatively small.